Analysis of by-pass capacitance in high speed PCB

- Sep 01, 2018-

1 the introduction

With the decrease of system volume, the increase of working frequency, and the complexity of system function, it needs many different embedded function modules to work at the same time.Only if each module has good EMC and low EMI can the whole system function be guaranteed.This requires the system itself not only to have a good performance of shielding from external interference, but also to work with other systems at the same time, without serious EMI.In addition, switching power supply is more and more widely used in the design of high-speed digital system.Not only the power system is vulnerable to interference, but also the noise generated during power supply will bring serious EMC problems to the whole system.Therefore, in the design of high speed PCB, how to remove power noise is the key to ensure good power integrity.This paper analyzes the filtering characteristics of capacitor, the influence of stray inductance capacitor's filtering performance, and the current loop phenomenon in PCB, and then makes some conclusions on how to choose bypass capacitor.In this paper, the generation mechanism of power source noise and ground spring noise is analyzed, and various placement modes of by-pass capacitance in PCB are analyzed and compared.

2. Insert loss characteristics, frequency response characteristics and capacitor filtering characteristics

2.1 insert loss characteristics of ideal capacitance

The ability of EMI power filter to suppress interference noise is usually measured by Insertion Loss characteristics.Insertion loss is defined as the ratio of noise source to noise power P2 at load after the noise source is transmitted to the load P1 and the filter is connected, which is expressed as dB (dB).FIG. 1 shows the insert loss characteristics of the ideal capacitance. It can be seen that the slope of the insert loss curve corresponding to the 1 maximum F capacitor is close to 20dB/10 octaves.

When the frequency increases, the insertion loss value of the capacitance increases. That is to say, the value of P1/P2 increases. This means that after the system passes the capacitor filtering, the noise transmitted to the load decreases and the capacity of the capacitor to filter out high-frequency noise increases.From the formula analysis of ideal capacitance, when the capacitance is constant, the higher the signal frequency, the lower the loop impedance, that is, the capacitance is easy to filter out the high-frequency components.The conclusion drawn from two aspects is the same.

Again observe different capacitance of the curve, in the case of low frequency, all kinds of capacitance of the insertion loss value is approximately the same, but with the increase of frequency, small insertion loss value of the capacitance increases the size of the larger capacitance slower, P1 / P2 value increase is slower, which means large capacitor is easier to filter out low frequency noise.Therefore, when we design the high-speed circuit board, we usually put a capacitance of 1 ~ 10 terminal F on the power access end of the circuit board to eliminate low frequency noise.A 0.01 ~ 0.1 capacitor is placed between the power supply and ground wire of each device on the circuit board to remove high-frequency noise.

Connected to the power source and to the capacitance between the impedance can be calculated by the formula:, capacitor filter filter stack in the power supply system is the purpose of the communication component, can be seen from the above formula, when the frequency of certain, the capacitance value, the greater the loop impedance of the smaller, the easier it is to such communication signal through the capacitance current to the ground plane, in other words, that seems to be the greater the capacitance value its filtering effect is better, in fact is not the case, because the actual capacitance does not have all the characteristics of the ideal capacitance.The parasitic component of the actual capacitance is formed when the capacitor plate and leads are constructed, and these parasitic components can be equivalent to the resistance and inductance in series on the capacitance, which is commonly referred to as the equivalent series resistance (ESR) and equivalent series inductance (ESL). The model is shown in the left part of figure 2.If the parasitic resistance of the capacitance is ignored, the model can be equivalent to the right half of figure 2.This capacitance is actually a series resonant circuit.In the actual circuit or PCB design, the existence of parasitic capacitor inductance will have a great impact on the filtering performance of capacitor. Therefore, the small parasitic inductance capacitance should be selected in the system design.

2.2 high frequency response characteristics of actual capacitors

As we know from section 2.1, the capacitance loop becomes a series resonant loop due to parasitic inductance.The resonant frequency is, where: L is equivalent inductance;C is the actual capacitance.As shown in figure 3, when the frequency is less than f0, the capacitance is presented.When the frequency is greater than f0, it presents as inductance.So a capacitor is more like a band-stop filter than a low-pass filter.The ESL and ESR of capacitance are determined by the construction of capacitance and the medium materials used, independent of capacitance.High frequency suppression is not enhanced by replacing large capacity of the same type of capacitance.The impedance of the same type of capacitors with larger capacity is smaller than that of capacitors with smaller capacity when the frequency is lower than f0. However, when the frequency is higher than f0, ESL determines that there is no difference in impedance.It can be seen that capacitors with lower ESL must be used to improve the performance of high frequency filtering.The effective frequency range of any kind of capacitor is limited, while for a system, there is both low frequency noise and high frequency noise, so different types of capacitors are usually connected in parallel to achieve a wider effective frequency range.

 

The capacitance model is used to analyze the circulation problem in PCB

Improper placement of power decoupling capacitors will result in a large current loop on the printed circuit board.In order to reduce noise, in the design of high-speed printed circuit board, there is a very important principle: reduce the area of signal current loop.In the past, we used to consider only the starting point, path and end point of current outflow, but seldom the return path of current.In high frequency circuit, power supply and ground is often thought to be equivalent, so the current flows out of the way and the return way will form a current loop, in the current loop, due to various reasons, such as the parasitic inductance capacitance, PCB wiring of the inherent inductance, etc., makes the impedance of the loop is not zero, the current flows through this loop will produce a potential difference, if the current is changing, it will produce radiation, have interference on the system.In order to filter power supply, bypass capacitance is often added between power supply and ground in circuit design. There are two main purposes to add bypass capacitance in the loop. One is to increase the capacity to store charge in the loop, so as to avoid the transient current being too large and the ground elastic noise.Secondly, the appropriate placement of by-pass capacitance can provide nearby ground loop for noise signal, reduce the area of current loop and thus reduce the inductance of loop.In the circuit with by-pass capacitance, the noise frequency to be filtered is usually high-frequency ac signal, so the circuit will still radiate outward.In order to reduce this radiation, we need to reduce the impedance of the circuit as much as possible, and the position of bypass capacitance must be properly placed.Figure 4 shows the large current loop due to improper placement of the filter capacitor.